The present invention generally relates to a novel process for the production of silicone polymers. More particularly, the present invention relates to a continuous process for the production of high molecular weight polydiorganosiloxane gums from cyclic polysiloxanes.
An early method for preparing silicone gums was provided by Marsden in U.S. Pat. No. 2,546,036. Marsden taught the art that polydimethylsiloxane gums can be prepared by treating low molecular weight liquid polydimethylsiloxanes with sodium hydroxide as a catalyst. At room temperature, from 2 to 10 or 15 days may be required to convert the dimethyl silicone liquid to an elastic gum having suitable properties. At temperatures on the order of from 50.degree. to 150.degree. C. the conversion can be effected in from 10 to 96 hours.
Warrick, in U.S. Pat. No. 2,634,252, disclosed an effective catalyst for preparing gums from the standpoint of speed, viscosity control and product quality. However, for use of the Warrick catalyst it has been considered necessary to eliminate linear siloxanes and react only cyclic siloxanes in the presence of the catalyst.
In both of the foregoing instances it was necessary that the reaction or process be carried out in a batch mode as the reaction time was excessive and/or equipment was not available which would allow for a continuous process. Accordingly, research efforts for reducing the amount of time or manpower necessary to manufacture silicone gums has centered on the use of alternate catalysts or on attempts to provide a continuous process for making such silicone gums. Examples of alternate catalysts well known to those skilled in the art include potassium hydroxide, potassium silanolate, and, as described in U.S. Pat. No. 3,433,765, the catalyst derived by the reaction of a tetra-alkyl ammonium hydroxide and a siloxane tetramer.
One example of utilizing an apparatus to reduce the time required to manufacture silicone gums is found in U.S. Pat. No. 3,803,084 to Schnurrbusch et al. Briefly, Schnurrbusch et al. teaches that high polymer polyorganosiloxanes are prepared by stirring oligomeric organosiloxanes and a catalyst in an upright container. The stirring is effected with a spiral stirrer disposed along the wall of the container, while the center of the container is free of conveyor means for free axial flow of the polymerizing material therethrough. Such apparatus reduces the time necessary to effect polymerization from 7 hours to about 5 hours.
U.S. Pat. No. 4,128,568 to Buchner et al. discloses a continuous process for the preparation of polydiorganosiloxanes. Diorganosiloxanes, preferably cyclic siloxanes, are reacted with substances which regulate the polymer chain length in the presence of acid or alkaline reaction catalysts at temperatures between 20.degree. and 180.degree. C. The invention is characterized in that the individual components are mixed mechanically in certain proportions, the mixing and the reaction taking place in a reactor consisting of several zones, and provided with stirring or conveying elements in which in the adjacent zones opposite flows of the material to be reacted are produced by appropriate arrangement of the stirring or conveying elements. According to Buchner et al., a reaction mixture, after a residence time of about four hours, produced a polymer with a viscosity of 100,000 centipoise at 25.degree. C.
Petersen, U.S. Pat. No. 4,250,290, describes a process for the continuous polymerization of polydiorganosiloxanes having a viscosity of up to about 1,000,000 centipoise at 25.degree. C. Briefly, the process involves mixing cyclopolysiloxanes with an effective amount of catalyst, passing such mixture into an apparatus having static elements which provide essentially uniform plug flow characteristics to the catalyzed mixture passing therethrough, controlling the polymer viscosity by adding to the reaction zone from 100 to 500 parts per million water as a chainstopper and neutralizing the catalyst when the reaction is completed.
From the foregoing it can be appreciated that completely satisfactory methods for carrying out the continuous polymerization of polydiorganosiloxanes, particularly polydiorganosiloxane gums, have not been successfully provided.